Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique in which a user transmits on many orthogonal frequencies (or subcarriers). The orthogonal subcarriers are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects. An orthogonal frequency division multiple access (OFDMA) system assigns subcarriers to different users, rather than one user.
As noted, the total bandwidth of an OFDM/OFDMA system is divided into orthogonal narrowband frequency units called subcarriers. The number of subcarriers, N, is equal to the FFT/IFFT block size N used in the system. The number of subcarriers used for data may be less than N if subcarriers at the edge of the frequency spectrum are reserved as guard subcarriers. No information is transmitted on guard subcarriers.
Within an OFDM/OFDMA system, a resource unit may be defined as one or more pre-determined subcarriers that may be either contiguous or distributed. A resource set then may be defined as one or more resource units. FIG. 4 illustrates an example of frequency reuse according to a conventional embodiment of an OFDM/OFDMA wireless network. In FIG. 4, three neighboring cells—Cell A, Cell B, and Cell C—of an OFDM/OFDMA wireless network are illustrated. In the frequency reuse approach used in the prior art, a resource set is allocated to a cell on a fixed basis. Thus, Cell A is allocated Resource Set A, Cell B is allocated Resource Set B, and Cell C is allocated Resource Set C.
The fixed frequency reuse schemes used in the prior art permanently reserve a frequency band for use in a given cell. In the fixed frequency reuse approach, the reserved resources that are not used in a given cell at a given time cannot be used by other cells. Thus, unused frequency units in Resource Set A cannot be used in Cell C. This is an inefficient use of spectrum resources.
Therefore, there is a need for improved OFDM and OFDMA wireless networks that make efficient use of the available spectrum. In particular, there is a need for multi-carrier networks that are not limited to fixed frequency reuse schemes.